OK, here's a couple thoughts on the subject:
On my Ford pickup, there's a support bar or stamping that is used to hold the spare under the rear gas tank.
Large bolt at one end (fixed in place), with another bolt like a giant eyebolt at the other.
Kinda copy that, but I gotta tell ya, it'll be heavy and pretty awkward....easier than getting it up/down from a roof, tho.
I've also seen a 'winch'-type setup on some Japanese-manufactured pickups.
Basically for them you use the lug wrench as a winch, and crank the tire up or down from where it's hung on a cable.
In the skoolie setup, adapt a winch from a boat trailer; use plastic-coated steel cable and the appropriate fasteners etc. to suspend the wheel assembly. Since a winch can easily hold 500 pounds, a bus wheel shouldn't be a big problem.
Hang the tire under the rear behind the pumpkin if you have the space.